Words related to *klei-
1550s, "mildness or gentleness shown in exercise of authority," from Latin clementia "calmness, gentleness," from clemens "calm, mild," related to clinare "to lean," often said to be from PIE root *klei- "to lean" + participial suffix -menos (also in alumnus). For sense evolution, compare inclined in secondary meaning "disposed favorably." But de Vaan is dubious on phonological grounds.
Clemency is exercised only toward offenders, being especially the attribute of those in exalted places having power to remit or lighten penalty. [Century Dictionary]
Earlier in same sense was clemence (late 15c.). Meaning "mildness of weather or climate" is 1660s (a sense also in Latin); clement (adj.) is older in both senses (late 15c. and 1620s respectively) but now is used only in negation and only of the weather.
late 14c., "one who lives under the patronage of another," from Anglo-French clyent (c. 1300), from Latin clientem (nominative cliens) "follower, retainer" (related to clinare "to incline, bend"), from PIE *klient-, a suffixed (active participle) form of root *klei- "to lean." The notion apparently is "one who leans on another for protection." In ancient Rome, a plebeian under the guardianship and protection of a patrician (who was called patronus in this relationship; see patron).
The meaning "a lawyer's customer" is attested from c. 1400, and by c. 1600 the word was extended to any customer who puts a particular interest in the care and management of another. Related: Cliency.
The relation of client and patron between a plebeian and a patrician, although at first strictly voluntary, was hereditary, the former bearing the family name of the latter, and performing various services for him and his family both in peace and war, in return for advice and support in respect to private rights and interests. Foreigners in Rome, and even allied or subject states and cities, were often clients of Roman patricians selected by them as patrons. The number of a patrician's clients, as of a baron's vassals in the middle ages, was a gage his greatness. [Century Dictionary]
late 14c., "horizontal zone of the earth's surface measured by lines parallel to the equator," from Old French climat "region, part of the earth," from Latin clima (genitive climatis) "region; slope of the earth," from Greek klima "region, zone," literally "an inclination, slope," thus "slope of the earth from equator to pole," from a suffixed form of PIE root *klei- "to lean."
Ancient geographers divided the earth into zones based on the angle of sun on the slope of the earth's surface and the length of daylight. Some reckoned 24 or 30 climates between Meroe on the upper Nile in Sudan and the mythical Riphaean Mountains which were supposed to bound the Arctic; a change of climate took place, going north, at a place where the day was a half hour longer or shorter, according to season, than the starting point. Others counted 7 (each dominated by a particular planet) or 12 (dominated by zodiac signs).
Change of temperature gradually came to be considered more important, and by late 14c. the word was being used in the sense "a distinct region of the earth's surface considered with respect to weather." The sense shift to "combined results of weather associated with a region, characteristic condition of a country or region with reference to the variation of heat, cold, rainfall, wind, etc.," is attested by c. 1600. Figuratively, of mental or moral atmosphere, from 1660s.
1580s, in the rhetorical sense ("a chain of reasoning in graduating steps from weaker to stronger"), from Late Latin climax (genitive climacis), from Greek klimax "propositions rising in effectiveness," literally "ladder," from suffixed form of PIE root *klei- "to lean."
Originally in rhetoric an arrangement of successive clauses so that the last important word of one is repeated as the first important word of the next, as in Romans v.3-5: "... but we glory in tribulations also: knowing that tribulation worketh patience; and patience, experience; and experience, hope: And hope maketh not ashamed ...." Compare anadiplosis. From the rhetorical meaning, the word evolved through "series of steps by which a goal is achieved," to "escalating steps," to (1789) "high point of intensity or development," a usage credited by the OED to "popular ignorance."
The meaning "sexual orgasm" is recorded by 1880 (also in terms such as climax of orgasm), and is said to have been promoted from c. 1900 by birth-control pioneer Marie Stopes (1880-1958) and others as a more accessible word than orgasm (n.).
1938, in biology, "a graded series of differences within a species," a back-formation from incline or from a Latinized form of Greek klinein "to slope, to lean" (from PIE root *klei- "to lean"). Earlier, but now obsolete, was a verb cline, from Middle English clinen "to bend, bow," from Old French cliner, from Latin clinare.
1620s, "bedridden person, one confined to his bed by sickness," from French clinique (17c.), from Latin clinicus "physician that visits patients in their beds," from Greek klinike (techne) "(practice) at the sickbed," from klinikos "of the bed," from kline "bed, couch, that on which one lies," from suffixed form of PIE root *klei- "to lean."
Also "one who defers baptism until the death-bed" (1660s). Sense of "private hospital" is from 1884, from German Klinik in this sense, itself from French clinique, via the notion of "bedside medical education, examination of a patient by an instructor in the presence of students." The modern sense thus reverses the classical one, in which the "clinic" came to the patient. General sense of "conference for group instruction in something" is from 1919.